Modern soft starters, as illustrated in FIG. 1, include a semiconductor power section for motor starting and a bridging section for carrying current during the operating phase. The bridging section may, for example, be formed by a mechanical contactor. The bridging section considerably reduces the electrical losses during the operating phase, thus allowing the appliance dimensions to be reduced and the cooling measures provided by the user to be reduced. In older soft starters, the bridging section comprises a contactor with an arc quenching chamber.
Suitable driving of the semiconductor section and the bridging section by way of a control section allows arc formation to be virtually avoided when the current changes from the semiconductor section to the bridging section, and vice versa. The bridging section can therefore also be designed without an arc quenching chamber, thus considerably reducing the costs, the dimensions and the weight. One embodiment of the mentioned design is explained in more detail in international application, PCT/EP2005/012441, which was not published prior to this application, but whose contents are hereby incorporated herein by reference, in their entirety.
The cooling effect can be further improved by at least one fan which blows a cooling air flow over the current path, and preferably also over the cooling measures. The fan is preferably sufficiently large to allow an adequate cooling air flow to be produced. Furthermore, the fan is preferably placed such that the air flow flows around and cools the current paths and cooling measures which enlarge the surface areas, as optimally as possible. The fan is therefore preferably fitted to the housing cover B of the soft starter.
In the event of a fault or if the drive for the semiconductor section by the control section does not operate accurately, it is possible for an arc to be formed in the bridging section. In order to prevent the flashover from damaging the fan, the fan must be further away from the bridging section, thus increasing the appliance volume, in particular the height. This is disadvantageous, because it can reduce the customer attractiveness of the soft starter.